In conventional offshore drilling operations from, a floating drilling vessel, a large diameter marine riser generally connects surface drilling equipment on the floating drilling vessel to a blowout preventer stack connected to a subsea wellhead located on the seabed. The marine riser is generally filled with drilling fluid (or “drilling mud”) so that a total hydrostatic pressure on a formation being drilled in the wellbore is determined by the hydrostatic pressure of the mud in the drilled wellbore (below the seabed) plus the hydrostatic pressure in the marine riser (above the seabed). Conventionally, mud pumps are utilized to intensify fluid pressure for use in drilling oil wells or in conditioning oil wells, such as fracturing, with extremely high pressure or abrasive fluids. Various mud pumps and pressure intensification pumps are already known to exist that employ various means to overcome the difficulties encountered during the prolonged pumping of high volume, high pressure and abrasive materials.
In the past, these mud pumps have employed an elongated shaft in the drive system of the mud pump. The elongated shaft generally has a relatively long length with respect to the diameter of the shaft. For example, a conventional shaft for a mud pump can have an overall length of over 4 feet and a shaft diameter of approximately 5 inches. So as to allow this relatively small diameter shaft to be properly driven, a hub is mechanically connected to an end of the shaft. This hub can then be connected to a sheave so that drive belt can be extended thereover and driven by an associated motor.
In the past, the standard mud pump shaft has been manufactured by General Electric and was particularly designed for use on locomotives. This shaft was readily available from General Electric at a relatively low cost. Since the number of the drive shafts that have been manufactured for locomotives greatly exceeds the number of drive shafts used for mud pumps, this readily available and relatively inexpensive shaft was particularly adapted for use on mud pumps. The shaft became a standard item as used with mud pumps, and other pieces of oil field equipment, such that the footholds for the shaft became relatively standardized.
FIGS. 1 and 2 illustrate the conventional mud pump shaft that is presently employed in oil field equipment. In FIG. 1, the shaft 10 has a relatively elongated configuration with a hub 12 affixed at one end thereof. The hub 12 is secured to a tapered surface 14 at the end of the shaft 10. The tapered end surface 14 is tapered so as to widen in diameter from end 16 toward the shoulder 18 of shaft 10. The tapered surface 14 is specifically configured so that the hub portion 12 can be securely affixed thereto.
The hub 12 extends radially outwardly from the tapered surface 14. The hub 12 also has tapered diameter 20 which will match with the tapered outer diameter of the tapered surface 14. Conventionally, the hub 12 will be heat shrink fit onto the tapered surface 14 of shaft 10. This form of fitting can also be termed “interference fit”. The hub 12 is heated so as to slightly expand. The hub 12 can be placed over the tapered surface 14 and then cooled. As a result, the hub 12 will be fixedly and securely attached to the tapered surface 14 of the shaft 10. The addition of hub 12 allows the standard locomotive shaft to be adapted for use in mud pump operations.
As can be seen in FIG. 1, the hub 12 has an end face 22 having a plurality of bolt holes 24 formed therein. The bolt holes 24 allow the hub 12 to be securely mounted within a sheave. The end 16 of the shaft 10 will be flush with the end face 22 of the hub 12.
FIG. 2 illustrates a cross-sectional view of the connection between the shaft 10 and the hub 12. In particular, FIG. 2 illustrates that the hub 12 is mechanically connected along interface 26 with the tapered surface 14 of the shaft 10. The shoulder 18 acts a limit to the positioning of the hub 12 on the end of the shaft 10. Various other shoulders 28, 30, 32 and 34 are formed along the length of the shaft 10. The shoulders 28, 30, 32, and 34 allow the shaft 10 to be placed into the standardized footholds within the mud pump. The opposite end 36 of shaft 10 has hole 38 formed therein.
Over the years, the shaft and hub configuration of FIGS. 1 and 2 has performed satisfactorily in the field. In the past, the shafts 10 have be driven by DC motors. Typically, the hub 12 would be connected to a chain or a gear driven hub. In these arrangements, there have been relatively few instances of slippage or disconnection between the hub 12 and the tapered surface 14 of the shaft 10.
In the past, mud pumps systems were commonly driven by the variable speed DC motors. However, due to increasing pump size and power requirements, AC induction motors are now used in conjunction with the mud pump systems. In fact, the various pieces of equipment that are used in offshore platforms are increasingly driven by AC power. AC power can generate 800 horsepower at 1100 rpms so as to produce an extremely high torque upon the shaft. Mud pumps have a variable power curve. The torque requirements range between 130% and 80% of nominal. Unlike DC motors, AC motors can almost instantaneously supply the power requirements of the mud pump. On oilfield equipment, these AC motors are used for mud pumps, drawworks, top drives, propulsion systems and rotary tables. Recently, following the increased use of AC motors, it has been found that the standard mud pump shaft/hub configuration shown in FIGS. 1 and 2 began to experience failures. Experimentation determined that the shaft fatigue failures appeared to be due to bending stresses and not from torsional vibrations. Extensive analysis on this failure situation revealed that the shaft failures were largely the result of the mechanical connection between the hub and the end of the shaft. Since the shaft is of relatively small diameter at its connection with the hub, the bending stress failure resulted because of the cantilever effect imparted upon the shaft by the strong connection between the hub, the attached sheave and the belt drive from the AC motor.
At present, KEVLAR (TM) belts are used to drive the mud pump. These belts are very strong. If these belts are over-tightened, they can result in a bent or stressed motor shaft. As a result, it has been critical that the proper belt tension be applied so as to limit the side-load on the motor shaft and to prevent slipping the sheave. Air cylinders have been used in the past so as to accurately tension belts within the specifications for the standard shaft (as shown in FIGS. 1–3). Additionally, shaft failure can also result from a lack of alignment of the shaft. If the motor shaft and the pump shaft are not parallel, the outer portion of the sheave can be loaded more than the inboard side. This can cause the effective moment arm to be beyond the center of sheave and increase the bending moment and stresses in the motor shaft.
FIG. 3 illustrates the standard locomotive shaft 10 as used in conjunction with mud pumps. Shaft 10 is particularly illustrated as having tapered surface 14 adjacent end 16. Tapered surface 14 will receive the shrink-fit hub thereon. In normal use, the shaft 10 would not be connected to a hub but would be joined to the drive system of a locomotive. As can be seen, the prior art shaft 10 is not configured for direct use as the shaft of a mud pump.
It is important to note that when there is a shaft failure, a great deal of consequential damage can occur. For example, repair and replacement of the shaft is required before the mud pump can continue to be used. Because of the importance of the mud pump, it is also necessary to have replacement shafts available in the event of such a failure. Many times, repair is virtually impossible since the broken connection between the hub and the shaft would greatly damage both the hub and the receiving surface of the shaft. Conventionally, the hub and the shaft would have to be replaced. Whenever repair and replacement must be carried out on an offshore oil rig, there is a great deal of down time for labor and equipment. This can be extremely expensive. As a result, it is extremely desirable to avoid any hub or shaft failure during the oil drilling operations.
It is an object of the present invention to provide a shaft for a mud pump which minimizes failure and fractures.
It is another object of the present invention to provide a shaft for a mud pump which avoids the mechanical connection between the hub and the end of the shaft.
It is an object of the present invention to provide a shaft for a mud pump which is adaptable to the existing footholds in existing mud pumps.
It is still an object of the present invention to provide a shaft for a mud pump that is easy to manufacture, easy to use and easy to install.
It is still a further object of the present invention to provide a shaft which is adaptable for use in conjunction with AC motors for other types of oilfield equipment such as drawworks, top drives, propulsion systems and rotary tables.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.